1. Field of the Invention
This invention relates generally to multiwavelength laser sources using novel nonlinear crystals for frequency conversion of solid state lasers. More particularly, this invention relates the generation of coherent radiations at ultraviolet, visible and infrared wavelengths which are selected by frequency converters for multiple industrial and surgical applications. This invention is particularly, but not exclusively, useful for ophthalmic surgery.
2. Prior Art
Various commercial lasers have been used for surgical treatments including gas lasers (such as CO.sub.2, excimer, argon, cu- vapor lasers), liquid lasers (dye lasers) and solid state lasers (such as YAG, semiconductor, Ti:sapphire lasers). Important parameters for laser technology include wavelength, energy (or fluence), intensity (or power) and repetition rate which are designed for the particular purpose being employed. For medical applications which are governed mainly by the laser beam's wavelength, there is a strong need for a multiple-purpose laser system in which multiwavelengths may be generated from a single laser unit. A variety of laser active media are currently used for the generation of different wavelengths suitable for particular surgical applications. Typical examples of these laser active media are: Nd:YAG (for 1.064 microns), Ho:YAG (for 2.1 microns), Er:YAG (for 2.94 microns) and excimer lasers (for ultraviolet, 193-351 nm). A dual-wavelength (at 532 nm and 1064 nm) solid state laser using a frequency-doubled Nd:YAG laser is currently used for surgical treatments such as gynecology. For ophthalmic surgery, YAG-based lasers with dopants of Nd, Ho, or Er with output wavelengths at approximately 1, 2 and 3 microns and excimer lasers (at 193 nm, 308 nm) have been used, where the Ho:YAG, Er:YAG and excimer lasers (at 193) are known as the candidates for refractive surgery using either external ablation (corneal tissue bond-breaking) or internal ablation (elastic alternation).
In light of the above, it is an object of the present invention to incorporate a frequency-converter using nonlinear crystals capable of producing a multiwavelength beam having all the wavelengths described above (from ultraviolet to infrared), where a particular surgical treatment may be achieved from a single laser system by switching the frequency-converter. In particular, the invention discloses a solid state laser at wavelength 210 nm or 213 nm which is a potential substitute for the argon fluoride excimer laser but has the advantages of lower-cost, smaller- size, less-maintenance, greater precision and, more importantly, the absence of toxic and hazardous materials. Another objective of the present invention is to produce a yellow laser with wavelength around 585-589 nm by using frequency mixing of two solid state lasers in lithium triborate (LBO) crystal. This yellow laser provides a variety of surgical applications which are currently being performed by krypton or copper vapor lasers. It is yet another object of the present invention to use optical parametric oscillation in nonlinear crystals, in which the disclosed laser system can produce tunable (1.5-4.5 microns) wavelengths covering medical applications which currently use holmium and erbium lasers.
Besides ophthalmic surgery, the multiwavelength solid state laser disclosed in this invention may be applied to many other medical surgeries such as laser angiosurgery, laser lithotripsy and laser neurosurgery. Among many potential industrial applications, the ultraviolet wavelength produced by the present invention provides a fast and precise tool for optical processing and micromachining.
Nonlinear crystals are the essential elements of the present invention. Efficiency is always the key issue of any frequency conversion technique (FCT) using nonlinear crystals. The FCTs used in this invention include second harmonic generation (SHG), fourth harmonic generation (4 HG) and fifth harmonic generation (5 HG) which converts a laser output into shorter wavelengths 1/2, 1/4 and 15 of the fundamental wavelength, respectively. The frequency conversion efficiency depends on both the laser and the nonlinear crystal parameters such as beam divergence, beam quality, focusing, beam walk-off and crystal damage threshold. Moreover, this invention also employs the optical parametric oscillation (OPO) process which converts the fundamental wavelength to longer but tunable wavelengths.
The present invention uses nonlinear crystals including beta barium borate (BBO), lithium triborate (LBO), potassium titanyl phosphate (KTP) and potassium niobate (KNbO.sub.3). Among these crystals, LBO is a new novel crystal suitable for high-power laser application and BBO is a unique crystal which provides the ultraviolet wavelength (shorter than 220 nm). The properties, applications and description of the frequency conversion techniques used in this invention were published by the inventor, J. T. Lin, in Optical and Quantum Electronics, Vol. 22, S383-S313 (1990); Optics Communication, Vol. 80, 159 (1990). Another object of the present invention is to integrate these nonlinear crystals into one single unit for multiple medical applications and, in particular, for ophthalmic surgery using the ultraviolet and mid-infrared wavelength produced by the all solid state multiwavelength laser.
Prior art U.S. Patents that for the making of beta barium borate can be seen in U.S. Pat. No. 4,897,249 for a Barium Borate Preparation and in U.S. Pat. No. 4,931,133 for a High Temperature Solution Growth of Barium Borate (BaB.sub.2 O.sub.2) Other U.S. patents suggesting the use of Barium Borate in a laser system can be seen in U.S. Pat. No. 4,809,291 for a Diode Pumped Laser and Doubling to obtain Blue Light which suggest the use of a number of non linear electro-optic doubler materials including beta barium borate and in U.S. Pat. No. 4,933,945 for an Arrangement for Converting the Frequency of a Laser Beam which suggests that if the frequency is to be tripled in the apparatus shown, then a BBO crystal can be used in lieu of the KTP crystal. In U.S. Pat. No. 4,879,722 for a Generation of Coherent Optical Radiation by Optical Mixing a diode pumped neodymium doped lasant material such as Nd:YAG is used with a nonlinear crystal in the same cavity and suggests the use of potassium titanyl phosphate (KTiOPO.sub.4) but also mentions other known nonlinear crystals as including beta barium borate. U.S. Pat. No. 4,884,277 also uses a diode pumped Nd:YAG laser having two or more nonlinear crystals in the same cavity and suggests beta barium borate as one of the crystals in which all of the crystals can be of the same material or may include different crystals used in combination.